I was quite pleased with the Audya harmonizer. It did everything I wanted and sounded great. I still think the Korg has the edge though.
It may just be the setup I use most. They are both excellent. Stand-alone harmonizers as a rule have more options and easier access to comtrols on the fly, but I don't know that they sound any better.
I haven't had the Audya for several years and I went from PA3x to PA900 about nine months ago. I was pleased with all three.
The Yamaha harmonizer starting with Tyros 4 was improved over previous models and is now usable, but it is harder to set up, more prone to picking up external sounds and distorting, and not as natural sounding to my aging ears.
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DonM